Continuous process for producing spacer-modified nano graphene electrodes for supercapacitors

ABSTRACT

A specific embodiment of the present invention is a process for continuously producing a porous solid film of spacer-modified nano graphene platelets for supercapacitor electrode applications. This process comprises: (a) dissolving a precursor material in a solvent to form a precursor solution and dispersing multiple nano graphene platelets into the solution to form a suspension; (b) continuously delivering and forming the suspension into a layer of solid film composed of precursor material-coated graphene platelets overlapping one another, and removing the solvent from the solid film (e.g., analogous to a paper-making, mat-making, or web-making procedure); (c) continuously converting the precursor material into nodules bonded to surfaces of graphene platelets to form a porous solid film composed of spacer-modified graphene platelets; and (d) continuously collecting the porous solid film on a collector (e.g., a winding roller). The roll of porous solid film (mat, paper, or web) can then be cut into pieces for used as supercapacitor electrodes.

This invention is based on the results of a research project sponsoredby the US DOE SBIR Program. The US government has certain rights on thisinvention.

FIELD OF THE INVENTION

The present invention relates generally to the field of supercapacitorsor ultracapacitors, and more particularly to the nano grapheneplatelet-based electrode and supercapacitors containing such anelectrode.

BACKGROUND OF THE INVENTION

Electrochemical capacitors (ECs), also known as ultracapacitors orsupercapacitors, are being considered for uses in hybrid electricvehicles (EVs) where they can supplement a battery used in an electriccar to provide bursts of power needed for rapid acceleration, thebiggest technical hurdle to making battery-powered cars commerciallyviable. A battery would still be used for cruising, but capacitors (withtheir ability to release energy much more quickly than batteries) wouldkick in whenever the car needs to accelerate for merging, passing,emergency maneuvers, and hill climbing. The EC must also storesufficient energy to provide an acceptable driving range. To be cost-and weight-effective compared to additional battery capacity they mustcombine adequate specific energy and specific power with long cyclelife, and meet cost targets as well. Specifically, it must store about400 Wh of energy, be able to deliver about 40 kW of power for about 10seconds, and provide high cycle-life (>100,000 cycles).

ECs are also gaining acceptance in the electronics industry as systemdesigners become familiar with their attributes and benefits. ECs wereoriginally developed to provide large bursts of driving energy fororbital lasers. In complementary metal oxide semiconductor (CMOS) memorybackup applications, for instance, a one-Farad EC having a volume ofonly one-half cubic inch can replace nickel-cadmium or lithium batteriesand provide backup power for months. For a given applied voltage, thestored energy in an EC associated with a given charge is half thatstorable in a corresponding battery system for passage of the samecharge. Nevertheless, ECs are extremely attractive power sources.Compared with batteries, they require no maintenance, offer much highercycle-life, require a very simple charging circuit, experience no“memory effect,” and are generally much safer. Physical rather thanchemical energy storage is the key reason for their safe operation andextraordinarily high cycle-life. Perhaps most importantly, capacitorsoffer higher power density than batteries.

The high volumetric capacitance density of an EC (10 to 100 timesgreater than conventional capacitors) derives from using porouselectrodes to create a large effective “plate area” and from storingenergy in the diffuse double layer. This double layer, created naturallyat a solid-electrolyte interface when voltage is imposed, has athickness of only about 1 nm, thus forming an extremely small effective“plate separation.” In some ECs, stored energy is further augmented bypseudo-capacitance effects, occurring again at the solid-electrolyteinterface due to electrochemical phenomena such as the redox chargetransfer. The double layer capacitor is based on a high surface areaelectrode material, such as activated carbon, immersed in anelectrolyte. A polarized double layer is formed at electrode-electrolyteinterfaces providing high capacitance. This implies that the specificcapacitance of a supercapacitor is directly proportional to the specificsurface area of the electrode material. This surface area must beaccessible by electrolyte and the resulting interfacial zones must besufficiently large to accommodate the so-called double-layer charges.

Experience with ECs based on activated carbon electrodes shows that theexperimentally measured capacitance is always much lower than thegeometrical capacitance calculated from the measured surface area andthe width of the dipole layer. For very high surface area carbons,typically only about ten percent of the “theoretical” capacitance wasobserved. This disappointing performance is related to the presence ofmicro-pores and ascribed to inaccessibility of some pores by theelectrolyte, wetting deficiencies, and/or the inability of a doublelayer to form successfully in pores in which the oppositely chargedsurfaces are less than about 2 nm apart. In activated carbons, dependingon the source of the carbon and the heat treatment temperature, asurprising amount of surface can be in the form of such micro-pores.

It would be desirable to produce an EC that exhibits greater geometricalcapacitance using a carbon based electrode having a high accessiblesurface area, high porosity, and reduced or no micro-pores. It would befurther advantageous to develop carbon-based nano-structures that areconducive to the occurrence of pseudo-capacitance effects, such as theredox charge transfer.

Carbon nanotubes (CNT) are nanometer-scale sized tube-shaped moleculeshaving the structure of a graphite molecule rolled into a rube. Ananotube can be single-walled or multi-walled, dependent upon conditionsof preparation. Carbon nanotubes typically are electrically conductiveand mechanically strong and stiff along their length. CNTs are beingstudied for electrochemical supercapacitor electrodes due to theirunique properties and structure, which include high specific surfacearea (e.g. up to 1,300 m²/g), high conductivity, and chemical stability.Capacitance values from 20 to 180 F/g have been reported, depending onCNT purity and electrolyte, as well as on specimen treatment such as CO₂physical activation, KOH chemical activation, or exposure to nitricacid, fluorine, or ammonia plasma. Conducting polymers, such aspolyacetylene, polypyrrole, polyaniline, polythiophene, and theirderivatives, are also common electrode materials for supercapacitors.The modification of CNTs with conducting polymers is one way to increasethe capacitance of the composite resulting from redox contribution ofthe conducting polymers. In the CNT/conducting polymer composite, CNTsare electron acceptors while the conducting polymer serves as anelectron donor. A charge transfer complex is formed between CNTs intheir ground state and aniline monomer. A number of studies onCNT/conducting polymer composites for electrochemical capacitorapplications have been reported.

However, there are several drawbacks associated with carbonnanotube-filled composites. First, CNTs are known to be extremelyexpensive due to the low yield, low production rate, and lowpurification rate commonly associated with the current CNT preparationprocesses. The high material costs have significantly hindered thewidespread application of CNTs. Second, CNTs tend to form a tangled messresembling a hairball, which is difficult to work with (e.g., difficultto disperse in a liquid solvent or resin matrix). This and otherdifficulties have significantly limited the scope of application ofCNTs.

Instead of trying to develop much lower-cost processes for making CNTs,researchers at Nanotek Instruments, Inc. have worked diligently todevelop alternative nano-scaled carbon materials that exhibit comparableproperties and can be mass-produced at much lower costs. Thisdevelopment work has led to the discovery of processes for producingindividual nano-scaled graphite planes (individual single-layer graphenesheets) and stacks of multiple graphene sheets, which are collectivelycalled nano graphene platelets (NGPs). A single-layer graphene sheet isbasically a 2-D hexagon lattice of sp² carbon atoms covalently bondedalong two plane directions. The sheet is essentially one carbon atomthick, which is smaller than 0.34 nm. The structures of NGPs may be bestvisualized by making a longitudinal scission on the single-wall ormulti-wall of a nano-tube along its tube axis direction and thenflattening up the structure to form a single-layer or multi-layergraphene platelet. In practice, NGPs are obtained from a precursormaterial, such as graphite particles, using a low-cost process, but notvia flattening of CNTs. These nano materials are cost-effectivesubstitutes for CNTs or other types of nano-rods for various scientificand engineering applications.

Nano graphene materials have recently been found to exhibitexceptionally high thermal conductivity, high electrical conductivity,and high strength. As a matter of fact, single-layer graphene exhibitsthe highest thermal conductivity and highest intrinsic strength of allcurrently known materials. Another outstanding characteristic ofgraphene is its exceptionally high specific surface area. A singlegraphene sheet provides a specific external surface area ofapproximately 2,675 m²/g (that is accessible by liquid electrolyte), asopposed to the exterior surface area of approximately 1,300 m²/gprovided by a corresponding single-wall CNT (interior surface notaccessible by electrolyte). The electrical conductivity of graphene isslightly higher than that of CNTs.

Two of the instant applicants (A. Zhamu and B. Z. Jang) and theircolleagues were the first to investigate NGP- and other nanographite-based nano materials for supercapacitor application [Please seeRefs. 1-5 below; the 1^(st) patent application was submitted in 2006 andissued in 2009]. After 2007, researchers began to realize thesignificance of nano graphene materials for supercapacitor applications[Refs. 6-12]

LIST OF REFERENCES

-   1. Lulu Song, A. Zhamu, Jiusheng Guo, and B. Z. Jang “Nano-scaled    Graphene Plate Nanocomposites for Supercapacitor Electrodes” U.S.    Pat. No. 7,623,340 (Nov. 24, 2009).-   2. Aruna Zhamu and Bor Z. Jang, “Process for Producing Nano-scaled    Graphene Platelet Nanocomposite Electrodes for Supercapacitors,”    U.S. patent application Ser. No. 11/906,786 (Oct. 4, 2007) now U.S.    Pat. No. 7,875,219.-   3. Aruna Zhamu and Bor Z. Jang, “Graphite-Carbon Composite    Electrodes for Supercapacitors” U.S. patent application Ser. No.    11/895,657 (Aug. 27, 2007) now U.S. Pat. No. 7,948,739.-   4. Aruna Zhamu and Bor Z. Jang, “Method of Producing Graphite-Carbon    Composite Electrodes for Supercapacitors” U.S. patent application    Ser. No. 11/895,588 (Aug. 27, 2007) now U.S. Pat. No. 8,497,225.-   5. Aruna Zhamu and Bor Z. Jang, “Graphene Nanocomposites for    Electrochemical cell Electrodes,” U.S. patent application Ser. No.    12/220,651 (Jul. 28, 2008).-   6. S. R. Vivekchand, et al., “Graphene-based Electrochemical    Supercapacitor,” J. Chem Sci., Vol. 120 (January 2008) pp. 9-13.-   7. M. D. Stoller, et al, “Graphene-based Ultracapacitor,” Nano    Letters, Vo. 8 (2008) pp. 3498-3502.-   8. X. Zhao, “Carbon Nanosheets as the Electrode Material in    Supercapacitors,” J. of Power Sources,” 194 (2009) 1208-1212.-   9. X. Zhao, “Supercapacitors Using Carbon Nanosheets as Electrode,”    US Pat. Pub. No. 2008/0232028 (Sep. 25, 2008).-   10. Y. Wang, “Supercapacitor Devices Based on Graphene    Materials,” J. Phys. Chem., C. 113 (2009) 13103-13107.-   11. H. Wang, et al., “Graphene Oxide Doped Polyaniline for    Supercapacitors,” Electrochem. Communications, 11 (2009) 1158-1161.-   12. Y. P. Zhang, et al. “Capacitive Behavior of Graphene-ZnO    Composite Film for Supercapacitors,” J. Electroanalytical Chem.,    634 (2009) 68-71.

However, these prior art workers have failed to recognize the notionthat individual nano graphene sheets have a great tendency to re-stackthemselves, effectively reducing the specific surface areas that areaccessible by the electrolyte in a supercapacitor electrode. Thesignificance of this graphene sheet overlap issue may be illustrated asfollows: For a nano graphene platelet with dimensions of l (length)×w(width)×t (thickness) and density ρ, the estimated surface area per unitmass is S/m=(2/ρ) (1/l+1/w+1/t). With ρ≅2.2 g/cm³, l=100 nm, w=100 nm,and t=0.34 nm (single layer), we have an impressive S/m value of 2,675m²/g, which is much greater than that of most commercially availablecarbon black or activated carbon materials used in the state-of-the-artsupercapacitor. If two single-layer graphene sheets stack to form adouble-layer NGP, the specific surface area is reduced to 1,345 m²/g.For a three-layer NGP, t=1 nm, we have S/m=906 m²/g. If more layers arestacked together, the specific surface area would be furthersignificantly reduced. These calculations suggest that it is essentialto find a way to prevent individual graphene sheets to re-stack and,even if they re-stack, the resulting multi-layer structure would stillhave inter-layer pores of adequate sizes. These pores must besufficiently large to allow for accessibility by the electrolyte and toenable the formation of double-layer charges, which typically require apore size of at least 2 nm.

Thus, it is an object of the present invention to providesurface-modified nano graphene sheets that naturally provide inter-layerpores when they stack or overlap with one another to form asupercapacitor electrode. The resulting electrode exhibits a highsurface area typically greater than 100 m²/gm, more typically greaterthan 300 m²/gm, even more typically greater than 500 m²/gm, and mosttypically greater than 1,000 m²/gm. In many cases, the specific surfacearea reaches the theoretical value of 2,675 m²/g, which translates intoan ultra high specific capacitance.

Surface modifications were achieved by using a spacer approach in whichnano-scaled spacer particles are either chemically bonded to orphysically attached to a surface of a graphene sheet. It may be notedthat although Zhang et al [Ref. 12] produced a hybrid graphene-ZnO filmas a supercapacitor electrode, the ZnO layer was a complete, continuousfilm, which was not in the form of discrete particles and could notserve as a spacer. In Zhang's report, ZnO was used to offer apseudo-capacitance effect, not a spacer.

SUMMARY OF THE INVENTION

One preferred embodiment of the present invention is a process ofproducing spacer-modified nano graphene platelets as a supercapacitorelectrode material. The modified platelet comprises: (a) a nano grapheneplatelet having a thickness smaller than 10 nm (preferably less than 1nm and most preferably less than 0.4 nm); and (b) discrete,non-continuous, and non-metallic bumps or nodules bonded to a surface ofthe graphene platelet to serve as a spacer. Preferably, there aremultiple bumps or nodules bonded to both surfaces of the platelet toeffectively increase a specific surface area of the platelet.

In one preferred embodiment, the process for producing spacer-modifiednano graphene platelets comprises: (a) dissolving a precursor materialin a solvent to form a precursor solution; (b) dispersing multiple nanographene platelets into the solution to form a suspension, wherein theplatelets have a thickness smaller than 10 nm; (c) forming thesuspension into a layer of solid film by removing the solvent from thesolid and allowing the precursor material to adhere or bond to a surfaceof the graphene platelets; and (d) thermally or chemically convertingthe precursor material into spacer nodules bonded to graphene plateletsurfaces. Multiple spacer-modified NGPs overlap naturally to form aclosely packed, but highly porous mat, web, or paper structure for useas a supercapacitor electrode.

The graphene platelets preferably comprise graphene platelets that havean average thickness no more than 2 nm or no more than 5 graphene layersper platelet and, most preferably, the platelets comprise mostlysingle-layer graphene to maximize the specific surface area of theresulting electrode. The precursor material contains a material selectedfrom the group consisting of petroleum pitch, coal tar pitch, polymer,resin, aromatic organic molecules, sol-gels, and combinations thereof.The precursor material can contain a precursor to the group of materialsconsisting of metal oxide, metal carbide, metal nitride, metal sulfide,metal halide, and combinations thereof.

The bumps or nodules contain a material selected from the groupconsisting of carbon, pitch, metal oxide, metal carbide, metal nitride,metal sulfide, metal halide, and combinations thereof. Preferably, thebumps or nodules contain a material selected from the group consistingof RuO₂, IrO₂, NiO, MnO₂, VO_(x), TiO₂, Cr₂O₃, CO₂O₃, PbO₂, Ag₂O,MoC_(x), Mo₂N, WC_(x), WN_(x), and combinations thereof. The spacercarbon may be produced by pyrolyzing a polymeric material coated onto asurface of the graphene platelet. Preferably, the spacer carbon isproduced by pyrolyzing a polymeric coating substance selected from thegroup consisting of phenol-formaldehyde, polyacrylonitrile, styrenedivinyl benzene, cellulosic polymers, polyfurfuryl alcohol,cyclotrimerized diethynyl benzene, and combination thereof.

The bumps or nodules preferably have a height no less than 1 nm, furtherpreferably no less that 2 nm. The main goal is for the spacer to helpcreate pores that are larger than 2 nm when multiple NGPs are stackedtogether.

The graphene platelet preferably has no more than 5 graphene layers,more preferably no more than 3 layers, and most preferably single-layerso that the modified nano graphene platelet exhibits a specific surfacearea greater than 500 m²/g, preferably greater than 900 m²/g, morepreferably greater than 1,384 m²/g, and most preferably greater than2,600 m²/g.

A specific embodiment of the present invention is a process forcontinuously producing a porous solid film of spacer-modified nanographene platelets as a supercapacitor electrode. This preferred processcomprises: (a) dissolving a precursor material in a solvent to form aprecursor solution and dispersing multiple nano graphene platelets intothe solution to form a suspension; (b) continuously delivering andforming the suspension into a layer of solid film composed of precursormaterial-coated graphene platelets overlapping one another, and removingthe solvent from the solid film (e.g., a paper-making procedure, amat-making procedure, or a web-making procedure); (c) continuouslyconverting the precursor material into nodules bonded to surfaces ofgraphene platelets to form a porous solid film composed ofspacer-modified graphene platelets; and (d) continuously collecting theporous solid film on a collector.

In one preferred embodiment, step (b) comprises delivering a controlledamount of suspension onto a moving substrate and forming the solid filmon the substrate and step (d) comprises collecting the porous solid filmon a winding roller. Step (c) may comprise heating said solid film, orcomprises chemically or thermally converting said precursor materialinto nodules of a material selected from the group consisting of carbon,metal oxide, metal carbide, metal nitride, metal sulfide, metal halide,and combinations thereof.

For convenience, the precursor solid film may be collected on a windingroller first and then, at a later time or at a different operatinglocation, the solid film may be heated to convert the precursor materialinto a spacer material (e.g. carbon). Hence, another preferredembodiment of the present invention is a process for continuouslyproducing a solid film of spacer-modified nano graphene platelets. Theprocess comprises: (a) dissolving a precursor material in a solvent toform a precursor solution and dispersing multiple nano grapheneplatelets into the solution to form a suspension; (b) continuouslydelivering and forming the suspension into a layer of solid filmcomposed of precursor material-coated graphene platelets overlapping oneanother, and removing the solvent from the solid film (e.g. apaper-making procedure, a mat-making procedure, or a web-makingprocedure); and (c) continuously collecting the solid film on acollector. The process preferably further comprises a step of heatingthe solid film to convert the precursor material into nodules bonded tographene platelet surfaces to form a porous solid film composed ofspacer-modified graphene platelets.

Preferably, step (b) comprises delivering the suspension onto a movingsubstrate and forming the solid film thereon, and step (c) comprisescollecting the porous solid film on a winding roller.

In any one of the aforementioned processes, one may choose to add adesired amount of a property-modifier material into the suspensionwherein the property modifier is selected from the group consisting ofcarbon nanotubes, carbon nano-wires, carbon nano-fibers, graphiticnano-fibers, carbon black, activated carbon, nano-wires, ceramic nanoparticles, polymer nano particles, nano particles of metal oxidesexcluding zinc oxide, particles of metal carbides, metal nitrides, metalsulfides, metal-halogen compounds, metal chalcogenides, and combinationsthereof.

Another embodiment of the present invention is an electrochemical cellelectrode comprising a surface-modified nano graphene platelet asdescribed above. This electrochemical cell may be a supercapacitor, alithium ion battery, a lithium metal battery, a lithium-air battery, analkali battery, or an alkaline battery.

A preferred embodiment of the present invention is a supercapacitorcomprising two electrodes, a porous separator disposed between the twoelectrodes, and electrolyte in physical contact with the two electrodes,wherein at least one of the two electrodes comprises a surface-modifiednano graphene platelet as defined above. Further preferably, bothelectrodes comprise such a modified graphene platelet.

Further, the electrochemical cell electrode preferably comprises aplurality of surface-modified nano graphene platelets as defined above,wherein multiple platelets form a stack having pores between platelets.The pores preferably have a size greater than 2 nm. Hence, anotherpreferred embodiment of the present invention is a supercapacitor,wherein at least one of the two electrodes comprises such a multi-NGPelectrode. Most preferably, both electrodes are of this type.

The surface-modified nano graphene platelet may be activated to form anactivated platelet, functionalized, or both activated and chemicallyfunctionalized. In one preferred embodiment, the surface-modified nanographene platelet is functionalized with one or more functional groupsselected from the group consisting of —SO₃, —R′COX, —R′(COOH)₂, —CN,—R′CH₂X, —OH, —R′CHO, —R′CN, wherein R′ is a hydrocarbon radical, andwherein X is —NH₂, —OH, or a halogen.

Still another embodiment of the present invention is a spacer-modifiednano graphene platelet electrode, comprising: (a) multiple nano grapheneplatelets having an average thickness smaller than 10 nm; and (b)discrete, non-metallic nano-scaled particles that are disposed betweentwo graphene platelets to serve as a spacer. The spacer particles areselected from the group consisting of carbon nanotubes, carbonnano-wires, carbon nano-fibers, graphitic nano-fibers, carbon black,activated carbon, nano-wires, ceramic nano particles, polymer nanoparticles, nano particles of metal oxides excluding zinc oxide, metalcarbides, metal nitrides, metal sulfides, metal-halogen compounds, metalchalcogenides, and combinations thereof. Preferably, spacer particlescover an area less than 50% (more preferably less than 30%) of a surfaceof a nano graphene platelet.

The platelets preferably comprise single-layer graphene. The electrodehas a specific surface area preferably greater than 500 m²/g, morepreferably greater than 900 m²/g, even more preferably greater than1,300 m²/g, and most preferably greater than 2,600 m²/g.

Another embodiment of the present invention is a supercapacitorcomprising two electrodes, a porous separator disposed between the twoelectrodes, and electrolyte in physical contact with the two electrodes,wherein at least one of the two electrodes is a spacer-modified nanographene platelet electrode as described above. Most preferably, bothelectrodes are spacer-modified nano graphene platelet electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A transmission electron microscopic image of NGPs without surfacemodification; graphene sheets significantly overlap one another toreduce the available surface area.

FIG. 2 SEM image of slightly surface-modified NGPs, showing some surfacenodules.

FIG. 3 An electrode structure comprising multiple surface-modified NGPsoverlapped together, but slightly separated due to the presence ofspacer nodules, resulting in the formation of electrolyte-accessiblepores.

FIGS. 4 (A) and (B) show the specific surface area and specificcapacitance of electrodes containing carbon nodule-modified NGPs plottedas a function of the percentage area of the NGP surface being covered bycarbon nodules. The correlation between the specific surface area andthe specific capacitance is shown in (C).

FIG. 5 Schematic of a continuous process for producing porous solid mat,web, or paper for supercapacitor electrode applications.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing detailed description of the invention taken in connection withthe accompanying drawing figures, which form a part of this disclosure.It is to be understood that this invention is not limited to thespecific devices, methods, conditions or parameters described and/orshown herein, and that the terminology used herein is for the purpose ofdescribing particular embodiments by way of example only and is notintended to be limiting of the claimed invention.

For the purpose of defining the geometry of an NGP, the NGP is describedas having a length (the largest dimension), a width (the second largestdimension), and a thickness. The thickness is the smallest dimension,which is no greater than 100 nm and, in the present application, nogreater than 10 nm (preferably no greater than 2 nm, more preferably nogreater than 1 nm, and most preferably no greater than 0.4 nm orsingle-layer graphene only). When the platelet is approximately circularin shape, the length and width are referred to as diameter. In thepresently defined NGPs, there is no limitation on the length and width,but they are preferably smaller than 10 μm and more preferably smallerthan 1 μm. We have been able to produce NGPs with length smaller than100 nm or larger than 10 μm.

Despite the fact that individual graphene sheets have exceptionally highspecific surface area, un-modified graphene sheets have a great tendencyto re-stack together or overlap one another, thereby dramaticallyreducing the specific surface area that is accessible by theelectrolyte. This phenomenon is illustrated in FIG. 1, wherein severalsingle-layer graphene sheets overlap and re-stack tightly. There is nodiscernable gap or pore between two graphene sheets overlapped together.FIG. 2 shows some multi-layer graphene platelets with their surfacesslightly modified, having some discrete nodules or bumps. FIG. 3 showsan electrode structure composed of multiple NGPs with their surfacesbonded with carbon nodules that cover approximately 25% of a surfacearea. Clearly, the surface-bonded nodules serve as a spacer that helpsto prevent individual graphene sheets from re-stacking, resulting ininter-sheet pores that are typically greater than 2 nm in size.

Hence, the present invention provides a surface-modified(spacer-modified) nano graphene platelet, comprising: (a) a nanographene platelet having a thickness smaller than 10 nm (preferably lessthan 1 nm and most preferably less than 0.4 nm); and (b) discrete,non-continuous, and non-metallic bumps or nodules bonded to a surface ofthe graphene platelet to serve as a spacer. Preferably, there aremultiple bumps or nodules bonded to both surfaces of the platelet toeffectively increase a specific surface area of the platelet.

A preferred embodiment of the present invention is a process forproducing spacer-modified nano graphene platelets as a supercapacitorelectrode material. The process comprises: (a) dissolving a precursormaterial in a solvent to form a precursor solution; (b) dispersingmultiple nano graphene platelets into the solution to form a suspension,wherein the platelets have a thickness smaller than 10 nm (preferablyless than 1 nm and most preferably less than 0.4 nm); (c) forming thesuspension into a layer of solid film by removing solvent from the solidand allowing the precursor material to adhere or bond to a surface ofthe graphene platelets; and (d) thermally or chemically converting theprecursor material into nodules or bumps bonded to graphene plateletsurfaces to serve as a spacer.

A preferred material for these bumps or nodules is carbon due to itsgood ability to bond to the graphene substrate and carbon is also knownto be capable of forming double layer charges near its interface withthe electrolyte. As an example, discrete carbon nodule-bonded graphenesheets may be obtained with any one of the following processes:

The first process entails:

-   -   (a) dispersing or immersing a laminar graphite material (e.g.,        natural graphite powder) in a mixture of an intercalant and an        oxidant (e.g., concentrated sulfuric acid and nitric acid,        respectively) to obtain a graphite intercalation compound (GIC)        or graphite oxide (GO);    -   (b) exposing the resulting GIC or GO to a thermal shock,        preferably in a temperature range of 600-1,100° C. for a short        period of time (typically 15 to 60 seconds), to obtain        exfoliated graphite or graphite worms (some oxidized NGPs with a        thickness<100 nm could be formed at this stage if the        intercalation/oxidation step was allowed to proceed for a        sufficiently long duration of time; e.g. 24 hours);    -   (c) re-dispersing the exfoliated graphite to a liquid medium        containing an acid (e.g., sulfuric acid), an oxidizing agent        (e.g. nitric acid), or an organic solvent (e.g., NMP) to form a        suspension. Stirring, mechanical shearing, or ultrasonication        can be used to break up graphite worms to form oxidized NGPs to        accelerate the dispersion step;    -   (d) optionally allowing the oxidized NGPs to stay in the liquid        medium at a desired temperature for a duration of time until the        oxidized NGPs are converted into individual single-layer        graphene oxide sheets dissolved in the liquid medium;    -   (e) dissolving a desired amount of polymer, resin, or pitch        (e.g., petroleum or coal tar pitch) in the liquid medium to form        a suspension;    -   (f) casting or delivering a controlled amount of the suspension        into a solid film (with solvent being removed, e.g. via        vaporizing), which is composed of polymer-, resin-, or        pitch-coated NGPs overlapping one another (preferably with a        resin-to-NGP weight ratio of 1/100 to 1/10);    -   (g) carbonizing the polymer, resin, or pitch at a temperature of        400-1,200° C. for typically 0.5 to 5 hours to form discrete        carbon nodules bonded onto NGP surfaces. The solid film becomes        a porous film, with multiple nodule- or spacer-bonded NGPs        stacking up over one another to form inter-NGP pores.

It may be noted that steps (a) to (c) are the most commonly used stepsto obtain graphene oxide platelets in the field. However, we weresurprised to observe that step (d) was capable of converting all NGPsinto single-layer graphene or graphene oxide sheets dissolved ordispersed in a liquid. Oxidized NGPs or GO platelets may be chemicallyreduced to recover conductivity properties using hydrazine as a reducingagent.

As a second example, the process includes:

-   (a) Preparing a suspension containing pristine nano graphene    platelets (NGPs) dispersed in a liquid medium using, for instance,    direct ultrasonication (e.g., a process disclosed by us in U.S.    patent application Ser. No. 11/800,728 (May 8, 2007));-   (b) dissolving a desired amount of polymer, resin, or pitch (e.g.,    petroleum or coal tar pitch) in the liquid medium to form a    suspension containing NGPs dispersed in a solution;-   (c) casting the suspension to form a layer of film and removing    (e.g. vaporizing) the liquid to form a solid film composed of    polymer-, resin-, or pitch-coated NGPs (e.g., resin-to-NGP weight    ratio of 1/100 to 1/10); and-   (d) carbonizing the polymer, resin, or pitch at a temperature of    400-1,200° C. for typically 0.5 to 5 hours to form a porous solid    film composed of discrete carbon nodule-bonded NGPs.

In both examples, the suspension preparation, casting (film forming),and conversion procedures can be carried out continuously. The resultingporous solid film can be collected onto a winding roller in a continuousmanner.

Hence, a specific embodiment of the present invention is a process forcontinuously producing a porous solid film of spacer-modified nanographene platelets as a supercapacitor electrode. This preferred processcomprises: (a) dissolving a precursor material in a solvent to form aprecursor solution and dispersing multiple nano graphene platelets intothe solution to form a suspension; (b) continuously delivering andforming the suspension into a layer of solid film composed of precursormaterial-coated graphene platelets overlapping one another, and removingthe solvent from the solid film (e.g., a paper-making procedure, amat-making procedure, or a web-making procedure); (c) continuouslyconverting the precursor material into nodules bonded to surfaces ofgraphene platelets to form a porous solid film composed ofspacer-modified graphene platelets; and (d) continuously collecting theporous solid film on a collector.

Step (b) may comprise delivering a controlled amount of suspension ontoa moving substrate and forming the solid film on the substrate and step(d) may comprise collecting the porous solid film on a winding roller.Step (c) may comprise heating the solid film, or comprises chemically orthermally converting said precursor material into nodules of a materialselected from the group consisting of carbon, metal oxide, metalcarbide, metal nitride, metal sulfide, metal halide, and combinationsthereof.

For convenience, the precursor solid film may be collected on a windingroller first and then, at a later time or at a different operatinglocation, the solid film may be heated to convert the precursor materialinto a spacer material (e.g. carbon). Hence, another preferredembodiment of the present invention is a process for continuouslyproducing a solid film of spacer-modified nano graphene platelets forsupercapacitor electrode applications. The process comprises: (a)dissolving a precursor material in a solvent to form a precursorsolution and dispersing multiple nano graphene platelets into thesolution to form a suspension; (b) continuously delivering and formingthe suspension into a layer of solid film composed of precursormaterial-coated graphene platelets overlapping one another, and removingthe solvent from the solid film (e.g. a paper-making procedure, amat-making procedure, or a web-making procedure); and (c) continuouslycollecting the solid film on a collector. The process preferably furthercomprises a step of heating the solid film to convert the precursormaterial into nodules bonded to graphene platelet surfaces to form aporous solid film composed of spacer-modified graphene platelets.Preferably, step (b) comprises delivering the suspension onto a movingsubstrate and forming the solid film thereon, and step (c) comprisescollecting the porous solid film on a winding roller.

A preferred embodiment of this invention may be illustrated in FIG. 5. Aroller 12 continuously provides a moving supporting substrate 14 ontowhich a controlled amount of suspension or paste 18 is deposited from adispenser means 16. The suspension or paste is moved through a pair ofcompacting rollers 22, 24 that serves to control the thickness of thepaste. Solvent begins to evaporate when the paste/suspension isdispensed out of dispenser 16 and continues to evaporate throughout thesubsequent steps. The compacted paste 26 goes through a heating zonethat serves to fully vaporize the solvent and convert a precursor resininto carbon nodules bonded on nano graphene platelet surfaces. Theresulting porous solid film (mat, web, or paper), optionally along withthe supporting substrate, is then collected on a winding roller 28. Thisis a roll-to-roll process that is amenable to mass production ofspacer-modified graphene-based supercapacitor electrode materials.

In order to prepare graphene sheets bonded with nodules of metal oxide,metal carbide, metal nitride, metal halide, or metal sulfide, one maychoose to use chemical vapor deposition (CVD), physical vapor deposition(PVD), plasma-enhanced CVD, sputtering, arc deposition, plasma arcdeposition, spray pyrolysis, and solution phase deposition. Forinstance, one may expose graphene sheets to a stream of silane andoxygen mixture gas in a high-temperature tube furnace (e.g. at atemperature of 600-900° C.) for a short period of time to depositsilicon oxide particles onto the NGP surfaces. Particle depositionprocesses are well-known in the art.

Hence, in a preferred embodiment, the bumps or nodules contain amaterial selected from the group consisting of carbon, pitch, metaloxide, metal carbide, metal nitride, metal sulfide, and combinationsthereof. Preferably, the bumps or nodules contain a material selectedfrom the group consisting of RuO₂, IrO₂, NiO, MnO₂, VO_(x), PbO₂, Ag₂O,MoC_(x), Mo₂N, WC_(x), WN_(x), and combinations thereof.

In one preferred embodiment, one may obtain a suspension containingmultiple NGPs dispersed in a solvent, a binder resin dissolved in thissolvent, and nano particles of the above-cited inorganic materials. Acombination of nano particles and a binder resin is an example of aprecursor material. The suspension is then cast into a thin film, whichbecomes a solid film when solvent is removed. The resulting film iscomposed of multiple NGPs bonded with nano particles via a binder resin.This procedure is preferably followed by a heat treatment to convert theresin into carbon (or converting a nano particle-resin combination intocarbon-bonded particles), creating additional pores between NGPs.Alternatively, it is possible to have precursor molecules or speciesdissolved or dispersed in the solvent. After or during solvent removal,these precursor molecules can then be thermally or chemically convertedinto nodules of metal oxides, carbides, nitrides, etc.

The spacer carbon may be produced by pyrolyzing a precursor polymericmaterial coated onto a surface of the graphene platelet. Preferably, thespacer carbon is produced by pyrolyzing a polymeric coating substanceselected from the group consisting of phenol-formaldehyde,polyacrylonitrile, styrene divinyl benzene, cellulosic polymers,polyfurfuryl alcohol, cyclotrimerized diethynyl benzene, and combinationthereof.

The bumps or nodules preferably have a height no less than 1 nm, furtherpreferably no less that 2 nm. The main goal is for the spacer to helpcreate pores that are larger than 2 nm when NGPs are stacked together.The graphene platelet preferably has no more than 5 graphene layers,more preferably no more than 3 layers, and most preferably single-layerso that the modified nano graphene platelet exhibits a specific surfacearea greater than 500 m²/g, preferably greater than 900 m²/g, morepreferably greater than 1,384 m²/g, and most preferably greater than2,600 m²/g.

When the platelets have an average length, width, or diameter no greaterthan 1 μm and average thickness no greater than 10 nm, the resulting NGPelectrode tends to have a surface area greater than 300 m²/gm. When theaverage NGP thickness is 2 nm or smaller, the resulting electrodetypically has a surface area greater than 500 m²/gm. A single-layergraphene exhibits a specific area greater than 1,600 m²/gm.

Another preferred embodiment of the present invention is aspacer-modified nano graphene platelet electrode, comprising: (a)multiple nano graphene platelets having an average thickness smallerthan 10 nm; and (b) discrete, non-metallic nano-scaled particles thatare disposed between two graphene platelets to serve as a spacer. Theseparticles are intentionally resided between two NGPs, but notnecessarily chemically bonded to any NGP surface.

The spacer particles are selected from the group consisting of carbonnanotubes, carbon nano-wires, carbon nano-fibers, graphitic nano-fibers,carbon black, activated carbon, nano-wires, ceramic nano particles,polymer nano particles, nano particles of metal oxides excluding zincoxide, metal carbides, metal nitrides, metal sulfides, metal-halogencompounds, metal chalcogenides, and combinations thereof.

These spacer particles may be added to an NGP solution or suspension asprepared according to one of the aforementioned processes. The resultingslurry may be subjected to mechanical shearing, stirring, orultrasonication to achieve a homogeneous mixing or dispersion of thespace particles. The slurry may then be cast onto a substrate, filteredthrough a porous Teflon membrane, or allowed to go through apaper-making process to form a sheet of porous film, mat, or paper,which is then used as an electrode. This process can be conducted in acontinuous manner.

These spacer particles have a size preferably greater than 1 nm and morepreferably greater than 2 nm, but most preferably between 2 nm and 200nm. The spacer particles preferably cover an area less than 50% of asurface of a nano graphene platelet, more preferably less than 30% of asurface of a nano graphene platelet.

Another embodiment of the present invention is a supercapacitorcomprising two electrodes, a porous separator disposed between the twoelectrodes, and electrolyte in physical contact with the two electrodes,wherein at least one, preferably both, of the two electrodes comprises aspacer-modified electrode as defined above.

The NGPs used in the aforementioned electrode may be subjected to thefollowing treatments, separately or in combination, before or after thesurface modification or spacer incorporation operation:

-   -   (a) chemically functionalized;    -   (b) coated or grafted with a conductive polymer;    -   (c) deposition with transition metal oxides or sulfides, such as        RuO₂, TiO₂, MnO₂, Cr₂O₃, and CO₂O₃, for the purpose of imparting        pseudo-capacitance to the electrode;    -   (d) subjected to an activation treatment (analogous to        activation of carbon black materials) to create additional        surfaces and possibly imparting functional chemical groups to        these surfaces. The activation treatment can be accomplished        through CO₂ physical activation, KOH chemical activation, or        exposure to nitric acid, fluorine, or ammonia plasma.

Conducting polymers, such as polyacetylene, polypyrrole, polyaniline,polythiophene, and their derivatives, are good choices for use in thepresent invention. These treatments are intended for further increasingthe capacitance value through pseudo-capacitance effects such as redoxreactions. Alternatively, transition metal oxides or sulfides such asRuO₂, TiO₂, MnO₂, Cr₂O₃, and CO₂O₃ can be deposited onto the NGP surfacefor pseudo-capacitance. Other useful surface functional groups mayinclude quinone, hydroquinone, quaternized aromatic amines, mercaptans,or disulfides. This latter class of functional groups also has beenshown to impart pseudo-capacitance to CNT-based supercapacitors.

The following examples serve to illustrate the preferred embodiments ofthe present invention and should not be construed as limiting the scopeof the invention:

Example 1 Graphene from Carbon/Graphite Fibers

Continuous graphite fiber yarns (Magnamite from Hercules) were cut intosegments of 5 mm long and then ball-milled for 48 hours. Approximately20 grams of these milled fibers were immersed in a mixture of 2 L offormic acid and 0.1 L of hydrogen peroxide at 45° C. for 60 hours.Following the chemical oxidation intercalation treatment, the resultingintercalated fibers were washed with water and dried. The resultingproduct is a formic acid-intercalated graphite fiber material containinggraphite oxide crystallites.

Subsequently, approximately ½ of the intercalated or oxidized fibersample was transferred to a furnace pre-set at a temperature of 600° C.for 30 seconds. The compound was found to induce extremely rapid andhigh expansions of graphite crystallites. The as-exfoliated graphitefiber is designated as Sample-1a. Approximately half of Sample 1-amaterial was subjected to de-oxygenation at 1,100° C. for 20 minutes ina nitrogen atmosphere to obtain Sample-1b.

A small amount of the two materials was separately mixed with an aqueousethanol solution to form two separate suspensions, which were subjectedto further separation of exfoliated flakes using a Cowles shearingdevice. Both graphite oxide platelets (Sample 1-a) and reduced GOplatelets (essentially NGPs) were found to be soluble and well-dispersedin this aqueous solution.

Two separate processes were conducted to prepare surface-modified NGPsand electrodes composed of spacer/NGP stacks. In one process, a smallamount of polyethylene oxide (PEO) was dissolved in the suspensioncontaining graphene oxide or reduced graphene platelets to obtain a 2%polymer solution. The suspension was subjected to a spray-dryingtreatment at approximately 100° C. to remove water and ethanol. ThePEO-coated NGP or GO platelets were then placed into a tube furnacepreset at 700° C. for two hours. Discrete carbon nodules were found tobe well-bonded to NGP and GO surfaces.

In another process, carbon black (CB) particles and multi-walled carbonnanotubes (CNT) were separately added into the GO and NGP solution,respectively, with a CB-to-GO ratio of 5/95 and CNT-to-NGP ratio of10/90. The resulting suspension was then cast into a glass substrate,followed by a liquid removal step. The dried mat was used as asupercapacitor electrode.

Example 2 Graphene from Sulfuric Acid Intercalation and Exfoliation ofMCMBs

MCMB 2528 microbeads were supplied by Alumina Trading, which was theU.S. distributor for the supplier, Osaka Gas Chemical Company of Japan.This material has a density of about 2.24 g/cm³; a particle size maximumfor at least 95% by weight of the particles of 37 microns; median sizeof about 22.5 microns and an inter-planar distance of about 0.336 nm.MCMB 2528 (10 grams) were intercalated with an acid solution (sulfuricacid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05)for 30 minutes. Upon completion of the reaction, the mixture was pouredinto deionized water and filtered. The intercalated MCMBs wererepeatedly washed in a 5% solution of HCl to remove most of the sulphateions. The sample was then washed repeatedly with deionized water untilthe pH of the filtrate was neutral. The slurry was spray-dried andstored in a vacuum oven at 60° C. for 24 hours. The dried powder samplewas placed in a quartz tube and inserted into a horizontal tube furnacepre-set at a desired temperature, 600° C. for 30 seconds to obtainSample 2-a. A small quantity of the sample was mixed with water andultrasonicated at a 60 W power for 10 minutes to obtain a suspension.

Nano particles of TiO₂ produced in house by a plasma arc process and asmall quantity of water-soluble PEO were added to this graphenesuspension with a TiO₂:PEO:NGP ratio of 5:5:90. The resulting suspensionwas then cast into a glass substrate, followed by a water removal step.The dried film was then subjected to a heat treatment at 700° C. in aflowing nitrogen environment for one hour to carbonize the polymer. Theresulting carbon serves as a conductive binder to bond the TiO₂ nanoparticles to NGP surfaces. The resulting porous mat with TiO₂ particlesserving as a spacer was used as a supercapacitor electrode.

Example 3 Oxidation, Exfoliation, and De-Oxygenation of Natural Graphite

Graphite oxide was prepared by oxidation of natural flake graphite withsulfuric acid, sodium nitrate, and potassium permanganate at a ratio of4:1:0.05 at 30° C. for 24 hours, according to the method of Hummers[U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of thereaction, the mixture was poured into deionized water and filtered. Thesample was then washed with 5% HCl solution to remove most of thesulfate ions and residual salt and then repeatedly rinsed with deionizedwater until the pH of the filtrate was approximately 7. The intent wasto remove all sulfuric and nitric acid residue out of graphiteinterstices. The slurry was spray-dried and stored in a vacuum oven at60° C. for 24 hours. The interlayer spacing of the resulting laminargraphite oxide was determined by the Debey-Scherrer X-ray technique tobe approximately 0.73 nm (7.3 Å), indicating that graphite has beenconverted into graphite oxide.

The dried, intercalated (oxidized) compound was placed in a quartz tubethat was inserted into a horizontal tube furnace pre-set at 1,050° C. toeffect exfoliation for 1 minute. The resulting exfoliated graphite wasdispersed in water and subjected to ultrasonication for 10 minutes toobtain a nano graphene oxide suspension. The suspension was spray-driedto recover NGP powder, which was then transferred to a rotating quartztube inside a tube furnace preset at 950° C. Silane gas was introducedinto the quartz tube, along with a small stream of oxygen for up to fiveminutes. It was observed that discrete silicon dioxide nodules werebonded onto graphene surfaces with such a short reaction time. In aseparate run, the chemical vapor deposition time was allowed to exceed15 minutes, leading to the formation of continuous silicon dioxide film,instead of discrete particles. A continuous film on an NGP surface isnot an effective spacer.

Example 4 Spacer-Modified NGPs Further Treated withPoly(3-methyl-thiophene)

Electronically conductive polymers by themselves are promisingsupercapacitor electrode materials because the charge process involvesthe entire polymer mass and they provide low equivalent seriesresistance for the electrode. When combined with an NGP-type substratematerial, the conducting polymer can impart pseudo-capacitance to theelectrode. One desirable conductive polymer selected waspoly(3-methyl-thiophene) (pMeT), particularly its α-doped variant.Poly(3-methyl-thiophene) could be synthesized by oxidative chemicalpolymerization technique using ferric chloride as a dopant in an inertatmosphere. However, we chose to prepare PMeT doped with differentanions electrochemically in the presence of tetra-alkyl-ammonium saltsusing a carbon nodule-bonded NGP mat prepared in Example 1 as anelectrode. The specific capacitance of an NGP mat, a spacer-modified NGPmat with no further treatment, and a spacer-modified NGP mat with a pMeTtreatment were found to be 78 F/g, 190 F/g, and 243 F/g, respectively.These data have clearly demonstrated that the presently invented spacerapproach is surprisingly effective in helping NGP-based electrodes toachieve a much higher capacitance as compared with NGPs without aspacer.

This impressive result was achieved with low-cost NGPs, as opposed toexpensive CNT-based materials. A multi-wallCNT/poly(3,4-ethylenedioxythiophene) composite, prepared by chemical orelectrochemical polymerization, when evaluated in 1 M H₂SO₄, 6 M KOH, or1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile,exhibited capacitance values of 60 to 160 F/g. However, CNT materialsare much more expensive.

Example 5 Transition Metal Halide as a Spacer

A number of transition metal halides bearing a 2,6-bis(imino)piridylligand, LMCl₂, where L=2,6-(ArNCCH₃)₂C₅H₃N and M=transition metal atom),have been synthesized (prior art). The manganese halide complex (M=Mn)was electrochemically deposited onto the surface of an NGP mat electrodein a water-containing acetonitrile electrolyte (containing 0.1 M oftetra-butyl-ammonium perchlorate). By adjusting the imposing currentdensity and reaction time one could readily form discrete particlesbonded onto NGP surfaces. With less than 10% by weight of manganesehalide particles, the specific capacitance of the NGP mat was increasedfrom 98 F/g to 189 F/g. Other transition metal oxides or sulfides canalso be used as a spacer as well as a source of pseudocapacitance.

Example 6 Effect of the Spacer Nodule Amount

Additional series of samples were prepared with NGPs prepared from aprocess similar to those described in Example 2. The NGP sheets werethen dispersed in acetone in which phenolic resin was added. The ratioof phenolic resin to NGP was varied to obtain different phenolicresin-coated NGPs. Surface-bonded carbon nodules were formed when thevarious phenolic resin-coated NGP samples were cured at 150-200° C. fortwo hours, followed by carbonization at 900° C. for one hour. NGPsbonded with carbon nodules that cover a range of graphene surface areaswere obtained. These surface nodule-modified NGPs were made into a paperform with NGPs being stacked up in an organized manner to make a porouselectrode (e.g., FIG. 3).

Supercapacitor cells with both electrodes made of these spacer-modifiedNGPs were prepared and evaluated. The results were summarized in FIG. 4.FIG. 4 (A) and FIG. 4(B) show that the specific surface area of theelectrode and the corresponding specific capacitance of thesupercapacitor cell increases initially with the nodule coverage area,reaches a peak at approximately 27.4% surface coverage, and then beginsto decay after that. This observation implies that there exists anoptimum spacer content to maximize the surface areas accessible by theelectrolyte. There is a clear correlation between the specificcapacitance and the specific surface area as shown in FIG. 4(C). Thespecific surface area based on double layer capacitance alone, without acontribution from redox-based pseudo-capacitance, reaches a range of200-300 F/g, which is much higher than what could be achieved withcarbon nano-tubes and nano graphene sheets without surface spacermodification. The presently invented spacer approach enables asupercapacitor designer to take full advantage of the high specificsurface area of graphene-based materials by overcoming the most severetechnical problem associated with this new class of nano materials forsupercapacitor applications (i.e. the tendency to overlap and re-stackwith one another and, hence, dramatically reduce the effective surfaceareas).

In conclusion, we have successfully developed a new and novel class ofspacer-modified nano graphene platelets that are superior supercapacitorelectrode materials. A supercapacitor can make use of this material inone or both of the electrodes. These NGP-based nano materials exhibitexceptionally high capacitance and electrical conductivity. Otherdesirable features of NGPs include chemical stability and low massdensity. They are also of lower costs compared with carbon nano-tubebased materials. Both NGPs and modified NGPs can be mass-producedcost-effectively.

We claim:
 1. A process for continuously producing a solid film ofspacer-modified nano graphene platelets or sheets for supercapacitorapplications, said process comprising: (a) dissolving a spacer precursormaterial in a solvent to form a precursor solution and dispersingmultiple nano graphene platelets or sheets into said solution to form asuspension; (b) continuously delivering and forming said suspension intoa layer of solid film composed of spacer precursor material-coatedgraphene platelets or sheets overlapping one another, and removing saidsolvent from said solid film; (c) continuously collecting said solidfilm on a collector; and (d) heating said solid film to convert saidspacer precursor material into discrete, non-continuous, and nonmetallicnodules that are bonded to surfaces of said graphene platelets or sheetsto form a porous solid film composed of spacer-modified grapheneplatelets or sheets wherein said nodules serve as a spacer that preventsre-stacking of said graphene platelets or sheets with one another andsaid nodules are selected from the group consisting of metal oxide,metal carbide, metal nitride, metal sulfide, metal halide, andcombinations thereof.
 2. The process of claim 1, wherein said grapheneplatelets comprise single-layer graphene.
 3. The process of claim 1,wherein said graphene platelets have an average thickness no greaterthan 2 nm or no more than 5 graphene layers per platelet.
 4. The processof claim 1, wherein said precursor material contains a precursor to thegroup of materials consisting of RuO₂, IrO₂, NiO, MnO₂, VO_(x), TiO₂,Cr₂O₃, Co₂O₃, PbO₂, Ag₂O, MoC_(x), Mo₂N, WC_(x), WN_(x), andcombinations thereof, and a binder resin.
 5. The process of claim 1,wherein said nodules have a height no less than 1 nm.
 6. The process ofclaim 1, wherein said nodules have a height no less than 2 nm.
 7. Theprocess of claim 1, further comprising a step of activating saidsurface-modified nano graphene platelets to form activated platelets. 8.The process of claim 1, further comprising a step of chemicallyfunctionalizing said spacer-modified nano graphene platelets to formfunctionalized platelets.
 9. The process of claim 1, wherein saidmodified nano graphene platelets exhibit a specific surface area greaterthan 2,600 m²/g.
 10. The process of claim 9, further comprising a stepof chemically functionalizing said activated nano graphene platelets toform functionalized/activated platelets.
 11. The process of claim 1wherein said step (b) comprises a paper-making procedure, a mat-makingprocedure, or a web-making procedure.
 12. The process of claim 1 whereinsaid step (b) comprises delivering said suspension onto a movingsubstrate and forming said solid film thereon and said step (c)comprises collecting said porous solid film on a winding roller.
 13. Theprocess of claim 1 wherein said step (a) further comprises adding aproperty-modifier material into said suspension wherein said propertymodifier is selected from the group consisting of carbon nanotubes,carbon nano-wires, carbon nano-fibers, graphitic nano-fibers, carbonblack, activated carbon, nano-wires, ceramic nano particles, polymernano particles, nano particles of metal oxides excluding zinc oxide,particles of metal carbides, metal nitrides, metal sulfides,metal-halogen compounds, metal chalcogenides, and combinations thereof.14. The process of claim 1, wherein said modified nano grapheneplatelets exhibit a specific surface area greater than 500 m²/g.
 15. Theprocess of claim 1, wherein said modified nano graphene plateletsexhibit a specific surface area greater than 900 m²/g.
 16. The processof claim 1, wherein said modified nano graphene platelets exhibit aspecific surface area greater than 1,384 m²/g.
 17. A process forproducing a porous solid film of spacer-modified nano graphene plateletsor sheets for use as a supercapacitor electrode, said processcomprising: (a) dissolving a spacer precursor material in a solvent toform a precursor solution and dispersing multiple nano grapheneplatelets or sheets into said solution to form a suspension; (b) formingsaid suspension into a layer of solid film composed of precursormaterial-coated graphene platelets or sheets overlapping one another,and removing said solvent from said solid film; (c) converting saidspacer precursor material into discrete, non-continuous, and nonmetallicnodules that are bonded to surfaces of said graphene platelets or sheetsto form a porous solid film composed of spacer-modified grapheneplatelets or sheets wherein said nodules serve as a spacer that preventsre-stacking of said graphene platelets or sheets with one another andsaid nodules are selected from the group consisting of metal oxide,metal carbide, metal nitride, metal sulfide, metal halide, andcombinations thereof; and (d) collecting said porous solid film on acollector.
 18. The process of claim 17 wherein said step (b) comprises apaper-making procedure, a mat-making procedure, or a web-makingprocedure.
 19. The process of claim 17 wherein said step (b) comprisesdelivering said suspension onto a moving substrate and forming saidsolid film thereon and said step (d) comprises collecting said poroussolid film on a winding roller.
 20. The process of claim 17 wherein saidstep (c) comprises heating said solid film.
 21. The process of claim 17wherein said step (a) further comprises adding a property-modifiermaterial into said suspension wherein said property modifier is selectedfrom the group consisting of carbon nanotubes, carbon nano-wires, carbonnano-fibers, graphitic nano-fibers, carbon black, activated carbon,nano-wires, ceramic nano particles, polymer nano particles, nanoparticles of metal oxides excluding zinc oxide, particles of metalcarbides, metal nitrides, metal sulfides, metal-halogen compounds, metalchalcogenides, and combinations thereof.